The present invention relates to an image sensor for use in a facsimile and an image scanner to convert an optical signal into an electric signal.
FIG. 2 is a sectional view of a photo-electro conversion element 8 in a conventional image sensor. A photo-electro conversion region 2 such as pn junction is formed in a semiconductor substrate 1. The photo-electro conversion region 2 is sequentially covered with an oxide layer 3, a nitride layer 5 and a resin layer 6. A portion other than the photo-electro conversion region 2 is covered with a light-shielding layer 4 made of, for example, aluminum so as to block incident light.
However, in the conventional image sensor, the thickness d.sub.1 of the oxide layer 3 and the thickness d.sub.2 of the nitride layer 5 are comparable to the wavelength of incident light, and the refractive indexes of semiconductor substrate 1, oxide layer 3, nitride layer 5 and resin layer 6 are different so that the reflected light and incident light create multi-interference with each other at the boundary surface between adjacent layers. Due to this multi-interference, transmittance of the incident light depends on the wavelength of incident light. Further, the light transmittance of the same wavelength is varied according to the thicknesses d.sub.1 and d.sub.2. In the actual semiconductor fabrication process, it is difficult to control the thicknesses d.sub.1 and d.sub.2 at constant dimensions so that the spectral sensitivity of the image sensor is varied. For this reason, the sensitivity of an individual image sensor is not fixed to a given value when an LED is utilized as a light source.